Mastitis is a significant disease in cattle breeding which cause enormous economic losses for cow breeders. Mastitis is divided into clinical and subclinical mastitis. Typical symptoms of clinical mastitis are swelling, redness, soreness of the mammary gland, flakes in the milk, or milk-not-like secretion. Subclinical mastitis has seen increased somatic cell count in milk. The phenotypic relationships among udder conformation traits, the incidence of clinical mastitis, and somatic cells were analysed in Czech Holstein cattle. Clinical mastitis was monitored on six farms in the Czech Republic for 14 years. There were evaluated cows during the first lactation. The incidence of clinical mastitis was monitored in seven lactation periods of 50 days and for the whole lactation. Somatic cell score was monitored in 10 test days after 30 days and, on average, for the whole lactation. The total number of cows was 17,622. The highest incidence of clinical mastitis was observed at the beginning of lactation. Traits that had the highest impact on mastitis were fore udder attachment and udder depth. Mastitis and somatic cell score increased with increasing depth of udder. There were proposed appropriate selection criteria for the selection of heifers. There were strong and tight Fore Udder Attachment, intermediate and shallower Udder Depth, high Rear Udder Height, deep Central Ligament, intermediate Udder Width, Middle of quarter Front Teat Placement, intermediate Teat Length, midpoint Rear Teat Placement, and intermediate Body Condition Score. Optimal measurement points are the same for optimal somatic cell count and clinical mastitis.